1. Field of the Invention
The present invention relates to a centrifugal fan and, more particularly, to a centrifugal fan installed in a combination ventilated hood and microwave oven.
2. Description of the Background Art
In general, a combination ventilated hood and microwave oven (or, OTR (Over the Range)) is mainly used at homes and performs a cooking function by using microwave of a magnetron installed within a microwave oven body. Foul air generated in cooking in the microwave oven is ventilated by using a centrifugal fan installed at a wall surface of a space portion of an upper side of the microwave oven body.
FIG. 1 is a vertical-sectional view showing the interior of a general combination ventilated hood and microwave oven. As illustrated, the combination ventilated hood and microwave oven includes: a main body 1 having a cavity (C) to receive a food stuff; a case 3 positioned to cover the main body 1 and having an exhaust path 2 at both sides thereof; and a centrifugal fan 10 installed at a rear upper portion of the main body 1.
A rotational tray 5 is installed inside the main body 1 to rotate the food stuff received inside the cavity (C).
A magnetron 6 is installed outside the main body 1 to radiate microwave.
An air suction opening 7 is formed at a lower surface of the case 3, communicating with the exhaust path 2, and an air discharge opening 8 is formed at a rear upper portion of the case 3, communicating with the exhaust path 2.
With the combination ventilated hood and microwave oven, a user opens a door (not shown), mounts a food stuff on the rotational tray 5, and presses down an operation button (not shown). Then, the rotational tray 5 is rotated and at the same time the magnetron 6 radiates microwave to cook the food stuff.
In case of cooking a food stuff by using a gas oven (not shown) installed at a lower side of the microwave oven, the user presses down a hood operation button. Then, the centrifugal fan 10 is operated to suck air through the air suction opening 7. The sucked air is discharged outwardly through the exhaust path 2 and air discharge opening 8.
FIG. 2 is a perspective view showing a bi-suction type centrifugal fan in accordance with a conventional art, FIG. 3 is a plan view showing the bi-suction type centrifugal fan in accordance with the conventional art, and FIG. 4 is a vertical-sectional view showing the bi-suction type centrifugal fan in accordance with the conventional art.
As illustrated, in the conventional bi-suction type centrifugal fan 10, a housing 12 is positioned at both sides of a driving motor 11, an air suction opening 12a is formed at one side and an air discharge opening 12b is formed at the other side.
An impeller 13 is rotatably installed inside the housing 12 so that air sucked through the air suction opening 12a can be discharged through the air discharge opening 12b while the impeller 13 is rotated by the driving motor 11.
The impeller 13 includes a plurality of blades 13a, a middle plate 13b for serving as a reinforcing member and being coupled to a shaft (not shown) of the driving motor 11 at its middle portion, and a rim 13c for connecting the blades 13a and being formed at both ends thereof.
As for the both sides of the housing 12, if it is assumed that a portion where the air suction opening 12a is a bell-mouth side and a portion adjacent to the driving motor 11 is a motor bracket side, a motor bracket 14 is coupled to the motor bracket side.
The motor bracket 14 is coupled to the housing 12 so that the impeller 13 can be positioned inside the housing 12, and the motor bracket 14 serves to support the housing 12 by the driving motor 11.
The motor bracket 14 is coupled to the housing 12 so that a cut-off portion 15 is formed at an end of the air discharge opening 12b, and a V-shaped bent portion 15a is formed at the central portion of the cut-off portion 15.
The motor bracket 14 includes an air suction opening 14a at its central portion so that the air suction opening 14a can suck air to cool the driving motor 11.
In the bi-suction type centrifugal fan constructed as described above, as power is applied to the driving motor 11, the driving motor 11 is rotated and the impeller 13 fixed at the driving motor 11 is also rotated. At this time, suction force is generated so that external air is sucked into the housing 12 through the air suction opening 12a. 
The air introduced through the air suction opening 12a of the housing 12 is discharged to the air discharge opening 12b by the rotational force of the impeller 13.
And then, the driving motor 11 is cooled by the air introduced through the air suction opening 14a of the motor bracket 14.
However, in the conventional bi-suction type centrifugal fan, when air sucked into the scroll housing is discharged through the air discharge opening by the rotation of the impeller, very severe noise occurs from a blade passing frequency (BPF) in proportion to the number of blades of the impeller. Such a noise is most severe at the cut-off portion 15 positioned near the blade 13a of the impeller 13.
In addition, as air is discharged through the air discharge opening 12b, a flow noise occurs, which is the most severe at the cut-off portion 15.
Moreover, since the air sucked into the housing 12 from the motor bracket side flows in whirls at the inner side of the motor bracket 14 with a plane shape, there is a technical difficulty in heightening a discharge air volume and reducing the noise.
Furthermore, when air introduced into the housing 12 is discharged through the air discharge opening 12b, the air flow is inclined at the bent portion 15a of the cut-off portion 15, resulting in increase in a noise. Such a noise is known to be more increased if the bent portion 15a of the cut-off portion 15 comes at the same vertical line with the middle plate 13b of the impeller 13.